Pulverizers for grinding different type materials are well known in the prior art. Pulverizers are also known as mills. Solid fossil fuels such as coal are one such material wherein there exists a need to grind the material in order to render the solid fossil fuel suitable for use in certain applications, although there are other materials such as gypsum, cement, minerals, etc. that need to be subjected to pulverization as well in order to permit their use in various applications. Fossil fuel fired power generation systems represent one such application in which it is desired to employ pulverized solid fossil fuel, e.g., coal as the fuel. Such a system is commonly referred to as a solid fossil fuel fired system. Pulverized solid fossil fuel firing is favored over other methods of burning solid fossil fuel because pulverized fossil fuel burns like gas and, therefore, fires are easily lighted and controlled.
For purposes of the discussion that follows, the solid fossil fuel fired systems referred to above typically consist of the following major operating components: a solid fossil fuel feeder, an apparatus for pulverizing solid fossil fuel, a distribution system for distributing the pulverized solid fossil fuel, a furnace in which the pulverized solid fossil fuel is to be burned, and the requisite controls for effecting the proper operation of the solid fossil fuel fired system. Of particular interest herein is the apparatus for pulverizing the solid fossil fuel, which will often be referred to hereinbelow as a solid fossil fuel pulverizer. Solid fossil fuel pulverizers are not new. They have been known to exist in the prior art for more than half a century. Furthermore, many improvements in the construction and/or mode of operation of solid fossil fuel pulverizers have been made during this period.
There are a number of features that it is advantageous for any solid fossil fuel pulverizer to possess, but particularly those which are designed for employment in a solid fossil fuel fired power generation system. Reference is had here to features such as reliability, low power consumption, minimum maintenance and high capacity. In addition, such a solid fossil fuel pulverizer advantageously should also be characterized by quiet operation, integrated lubrication systems, convenient adjustment and control of solid fossil fuel flow and fineness, and the ability to handle the high temperature air that is required for high moisture solid fossil fuel.
One particular type of conventional solid fossil fuel pulverizer is commonly referred to in the industry as a bowl mill. This solid fossil fuel pulverizer obtains its name by virtue of the fact that the pulverization, i.e., grinding, of the solid fossil fuel which takes place therein is effected on a grinding surface that in configuration bears a resemblance to a bowl. Reference may be had by way of exemplification to U.S. Pat. No. 3,465,971, which issued Sep. 9, 1969 to J. F. Dalenberg et al., and/or U.S. Pat. No. 4,002,299, which issued Jan. 11, 1977 to C. J. Skalka, both of the patents being assigned to the same assignee as the instant application, for a teaching of the nature of the construction and the mode of operation of a prior art form of bowl mill that is suitable for use in a solid fossil fuel fired power generation system to effectuate the pulverization of the solid fossil fuel that is to be burned as fuel therein.
As taught by the aforereferenced patents, and as is understood in the art, a bowl mill essentially consists of a body portion in which a grinding table is mounted for rotation, typically three grinding rollers each mounted on a suitably supported journal, also referred to herein as grinding journals, that interact with the grinding table to effect the grinding of material interposed therebetween, material supply means for feeding to the interior of the bowl mill the material that is to be pulverized, and air supply means for supplying to the interior of the bowl mill the air required in the operation of the bowl mill. In accordance with the mode of operation of such a bowl mill, the material, which enters the bowl mill, is pulverized by virtue of the interaction of the grinding rollers with the grinding table. After being pulverized, the particles of material are thrown outwardly by centrifugal force whereby the particles of material are fed into a stream of warm air that is entering the bowl mill. The stream of air with the particles of material entrained therein flows into a classifier in which coarse particles of material are removed from the air stream. These removed coarse material particles are then returned to the grinding table for further pulverization, while the fine particles of material are carried through the bowl mill in the air stream, and exit along with the air.
A typical bowl mill, under actual operating conditions, when employed in a solid fossil fuel fired system has a capacity to pulverize approximately 200,000 pounds of solid fossil fuel per hour, i.e., has a throughput of approximately 200,000 pounds of solid fossil fuel per hour. One manner in which to increase bowl mill throughput, i.e., increase efficiency, is to increase the speed at which the grinding table rotates. However, too great an increase in rotational speed results in unstable operation, known as a rumble mill condition, that does not result in an increase in throughput. That is, with an increase in rotational speed comes an increase in centrifugal force, throwing partially pulverized material off the grinding table. The partially pulverized material must be returned to the grinding table multiple times before the partially pulverized material is sufficiently pulverized. Also, operating at higher rotational speeds increases the power used by the bowl mill.
Other problems with known bowl mills relate to the condition of the material on the grinding table, often referred to as the material bed. In many bowl mills the material bed is not controlled for effective grinding. That is, the material on the grinding table to be provided to each grinding roller is not uniform across the surface of the grinding table, with some surface areas of the grinding table perhaps not having any material thereon, and with other surface areas of the grinding table having differing depths of material thereon. Also, material that has been pulverized by the grinding rollers sometimes adheres to the grinding table surface. This adhered material is not timely removed from the bowl. As a result, the adhered material is over pulverized by the grinding rollers.
One solution to the problem of increased centrifugal force associated with increased rotational speed is found in U.S. Pat. No. 6,113,015, which issued on Sep. 5, 2000 to Brundick and which is assigned on its face to Loesche GmbH. In the Brundick invention a damming roll is included in between each grinding roller. Material moving toward the outer edge of the surface of the grinding table, i.e., the dam ring, due to centrifugal force is blocked by the damming rolls. The damming rolls do not impart grinding force on the material that is to be pulverized. Rather, the purpose of the damming rolls is to keep the material on the grinding table surface, in opposition to the centrifugal force. The retaining of the material on the grinding table surface gives rise to at least one problem. In addition to coarse particles, the damming rolls also keep fine particles of materials on the grinding table surface, resulting in an over pulverization of the material.
U.S. Pat. No. 4,981,269, which issued on Jan. 1, 1991 to Tadayuki Koga et al. and which is assigned on its face to Ube Industries, Ltd., is directed to problems associated with the material bed, discussed above. In the Koga invention auxiliary rollers, also known as slave rolls, are arranged alternatively with the grinding rollers. These slave rolls condition, i.e., compact and smooth, the material bed for effective grinding. That is, the slave rolls operate to level the material on the grinding table surface before it encounters a grinding roller. Also, U.S. Pat. No. 4,234,132, which issued on Nov. 18, 1980 to Theodore V. Maliszewski et al. and which is assigned to the same assignee as the instant application, discloses a stand alone air deflector for leveling the material on the grinding table surface so as to present a uniform layer of material to the grinding rollers. That is, the deflector channels air over the surface of the grinding table to impart a leveling force on the material bed.
U.S. Pat. No. 3,556,419, which issued on Jan. 19, 1971 to Gustav Frangquist and which is assigned to the same assignee as the instant application, is directed to remedying the adhesion of material to the surface of the grinding table. In the Frangquist invention rakes are arranged alternatively with the grinding rollers. Each rake includes three tines for breaking up material that is adhered to, i.e., caked on, the grinding table surface.
No existing techniques are known which both address the problems associated with increased centrifugal force and the problems associated with the condition of the material bed discussed above. That is, the Brundick invention addresses the problems associated with increased centrifugal force, but not the problems associated with the condition of the material bed, and the Koga, Maliszewski, and Frangquist inventions address unique aspects of the problems associated with the condition of the material bed, but not the problems associated with increased centrifugal force. Accordingly, a need exists for a high efficiency pulverizer which overcomes all of the problems associated with the prior art discussed above.